Compared with other macromolecular materials, fluororesins have lower dielectric constants, small dielectric tangents, and other excellent electrical characteristics. For this reason, fluororesins have been used in the past as dielectric materials, and have recently been made into foams in order to improve the electrical characteristics. Of these fluororesin materials, foams with closed-cell pore structures have recently attracted particular attention because of the advantage that such cells are crushed less readily than open cells.
In the past, fluororesin foams have been manufactured by methods in which a liquefied gas (e.g., hydrocarbon or fluorocarbon) is injected into a molten resin, and foaming is effected through the expansion of the gas contained in the resin as it emerges from the die. The reason that such methods have been adopted is that the molding temperature of a fluororesin is higher than that of an ordinary thermoplastic resin, so foaming cannot be accomplished using a thermally decomposable chemical foaming agent employed with such a thermoplastic resin.
However, the viscoelasticity of molten thermoplastic fluororesins is low and when a thermoplastic fluoropolymer resin foam is manufactured by extrusion molding, the gas that is formed by the decomposition of the foaming agent in the molten fluororesin causes rapid expansion due to the pressure difference with the atmosphere as the resin is extruded from the die orifice and released from the pressure that exists inside the extruder. As a result, the cells inside the fluororesin foam thus obtained are generally large, with an average diameter of approximately 150 micrometers or so, and the dispersion in the cell diameter is considerable. Thus, if such a fluororesin foam is used, for example, as an insulator for a thin insulated electrical wire, the cells will sometimes reach the outer surface of the insulator, making its surface rough or its outside diameter nonuniform, and causing the electrical characteristics to become nonuniform along the length of the electrical wire.